The present invention relates to a rechargeable power tool that includes a direct current motor which is rotated by electric power from a battery (a rechargeable battery) as a power source and controls rotation speed of the direct current motor in accordance with an operation state of a speed setting switch such as a trigger switch and others.
A conventionally known rechargeable power tool of this type includes a bridge circuit (in other words, an inverter circuit) which is constituted by switching devices (high-side switches and low-side switches), each provided between each of terminals of the direct current motor and a positive terminal and a negative terminal of a direct current power source (a battery), as a drive circuit for the direct current motor.
In the rechargeable power tool constituted as above, when a user operates a trigger switch as a speed setting switch, a controller sets a driving duty ratio for PWM controlling the direct current motor based on an operation amount of the trigger switch, and turns on/off the switching devices in the bridge circuit in accordance with the driving duty ratio, thereby controlling a rotation of the direct current motor.
Also known is a rechargeable power tool provided, other than the above-described trigger switch, with a speed selector as a speed setting switch which can switch a rotation speed of the direct current motor, for example, between two modes, that is, a high speed mode and a low speed mode. In the rechargeable power tool constituted as such, when a user sets the speed selector either to a high speed mode or to a low speed mode (switching operation) and draws the trigger switch, the controller controls the direct current motor such that the rotation speed of the direct current motor becomes a rotation speed corresponding to either of the selected speed modes. A rotational driving force of the direct current motor is transmitted to a tool output shaft directly or via a deceleration mechanism and so on.
In a rechargeable power tool, when an abnormal state, like mechanical locking of the direct current motor, malfunction of the controller, or short circuit of the direct current motor as a load, is generated during driving of the direct current motor as a power source, an excess current may flow into the direct current motor and the drive circuit therefor, thereby causing these components to be heated and burnt out in some cases.
Accordingly, the rechargeable power tool usually includes a protection circuit. The protection circuit determines whether or not the rechargeable power tool is in the above-described abnormal state during driving of the direct current motor. When it is determined that the rechargeable power tool is in an abnormal state, the protection circuit performs protective operation like stopping the driving of the direct current motor.
The protection circuit may be configured in various manners. For example, the protection circuit may include a current detector that detects a current value of the direct current motor. When the current value exceeds a certain threshold, it is determined that the direct current motor is in an abnormal state (an excess current is generated).
In recent years, however, various methods are proposed for detecting an excess current without providing the current detector in order to achieve miniaturization of the circuit and cost reduction. An example of such methods is disclosed in Unexamined Japanese Patent Publication No. 5-174874 in which when a monitored battery voltage is decreased by generation of an excess current due to a load short circuit, application of current to a load is stopped (switching devices provided on a current path are turned off).